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  <div class="question_difficulty">
   难度：Medium
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   <h1 class="question_title">
    114. Flatten Binary Tree to Linked List
   </h1>
   <p>
    Given a binary tree, flatten it to a linked list in-place.
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   <p>
    For example, given the following tree:
   </p>
   <pre>
    1
   / \
  2   5
 / \   \
3   4   6
</pre>
   <p>
    The flattened tree should look like:
   </p>
   <pre>
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6
</pre>
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   <h1 class="question_title">
    114. 二叉树展开为链表
   </h1>
   <p>
    给定一个二叉树，
    <a href="https://baike.baidu.com/item/%E5%8E%9F%E5%9C%B0%E7%AE%97%E6%B3%95/8010757" target="_blank">
     原地
    </a>
    将它展开为链表。
   </p>
   <p>
    例如，给定二叉树
   </p>
   <pre>    1
   / \
  2   5
 / \   \
3   4   6</pre>
   <p>
    将其展开为：
   </p>
   <pre>1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6</pre>
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